Houses core instrumentation. It differs from third-party because we are still monkey patching code but emitting it over tracing channel.  

Registering a core subscriber entails both instrumenting via the `instrument` method and providing the necessary parameters to the extended subscriber class for subscribing and creating the necessary telemetry.

This is an example for a core subscriber. That wraps the methods `bar` and `baz` on `core-lib-name`. Most core libraries simply create segments with a naming conventions of `<pkg-name>.<method>`. The `end`, `asyncEnd` events simply just touch the active segment.

Set `internal = true` when the instrumented methods delegate to one another (as `child_process.exec` calls `execFile`). Without it, one logical operation creates a segment per method, and because the async context then points at the inner segment, the outer one's timer is never stopped when the operation completes.

```js

const BaseCoreSubscriber = require('#agentlib/subscribers/core/base.js')
// eslint-disable-next-line n/no-unsupported-features/node-builtins
const { tracingChannel } = require('node:diagnostics_channel')
const shimmer = require('#agentlib/shimmer.js')
const instrumentedMethods = ['bar', 'baz']

class FakeCoreSubscriber extends BaseCoreSubscriber {
  constructor({ agent, logger }) {
    super({ agent, logger, packageName: 'core-lib-name', instrumentedMethods })
  }

  instrument(moduleBeingInstrumented) {
    const self = this
    shimmer.wrapMethod(
      moduleBeingInstrumented,
      this.packageName,
      instrumentedMethods,
      function wrapMethod(original, method) {
        const channel = tracingChannel(`${self.id}:${method}`)
        return function wrappedMethod(...args) {
          const data = { name: `${self.packageName}.${method}` }
          return channel.traceCallback(original, -1, data, this, ...args)
        }
      }
    )
  }
}

module.exports = FakeCoreSubscriber
```

Register the subscriber by adding an entry to `config.js`, which follows the same
shape as the non-core configs. `instrumentations` is empty because core
subscribers patch with `shimmer` rather than through Orchestrion channels.

```js
module.exports = {
  'core-lib-name': [
    { path: './core/core-lib-name', instrumentations: [] }
  ]
}
```

More to come as we migrate more libraries.